1. Field of the Invention
The disclosed technology relates to magnetic compasses and their calibration.
2. Description of the Related Technology
Generally, a compass is used to orientate the user with respect to the earth. Such orientation can be used for navigation or to convert measurements into a fixed reference frame. Multiple measurements, once converted into a fixed reference frame, can be compared, combined, or otherwise used together. A magnetic compass often determines the strength of a magnetic field, in geographic coordinates, at the location of the compass. In most cases, the determined magnetic field will be that of the earth. This information can be used to, for example, determine magnetic north. In another application, a magnetic compass simply indicates the direction of magnetic north.
Several challenges exist when calibrating a magnetic compass. For example, the compass may be on a relatively unstable platform, such as a small boat, during the calibration. In general, the quality of the calibration is reduced in highly dynamic conditions, such as when the platform is in motion during the calibration. In one application, the velocity of a given volume of water is desired referenced to a fixed coordinate system, e.g. geographic coordinates. Because the measuring platform may move in relation to this coordinate system (e.g., roll, pitch, yaw, and relative velocity), measurements made by the platform in its own coordinate system can be transformed to the fixed coordinate system.
As another example, local distortions of the earth's magnetic field may be present as a result of nearby magnetic minerals, such as basalt, or from solar activity. Further, magnetic north does not coincide with geographic (polar) north. Local perturbations from the substantially uniform magnetic field of the earth are generally classified as either hard iron effects or soft iron effects. Hard iron effects are measured as an offset from the earth's magnetic field, whereas soft iron effects are measured as a rotation and/or scaling of the earth's magnetic field.
Some magnetic compasses include a magnetometer and an accelerometer. A magnetometer senses and provides information regarding magnetic fields. An accelerometer senses and provides information regarding acceleration. By combining these measurements, the difference between a fixed reference frame, such as the earth, and a local reference frame, such as a vessel, can be determined.
Accordingly, a need exists for improved calibration methods.